Fuel cells are highly efficient devices for electric power production. There are several families of fuel cells, classified on the basis of the electrolyte material that supports ionic transport between the electrodes: phosphoric acid, alkaline, molten carbonate, solid oxide, and polymer electrolyte membrane (PEM). Of these, the PEM is the most promising for transportation applications, but there is a requirement that the electrolyte be maintained at a relatively high hydration state for optimal performance. Since the cells must operate at a temperature where the water vapor pressure is below the system pressure so that the electrolyte does not dry out, two-phase flow of both liquid and gas in the flow-fields is inevitable.
Current flow-field technology for phosphoric acid, alkaline and PEM fuel cells typically consists of grooved, serpentine, flow-fields through which the reactants flow (see, e.g., U.S. Pat. No. 5,108,849). An alternative flow-field is described in U.S. Pat. No. 4,769,297, where the flow-fields are formed from porous ribbed plates and the flow channels form a waffle-like structure and there is no separation of inlet and outlet channels. In order to bridge the open faced channels of the flow-field, a relatively rigid structure is required to facilitate the transition from the flow-channel to the catalyst layer adjacent the membrane that catalyzes the chemical reactions producing an electron current. This structure is the gas diffusion layer, or backing, and typically consists of a carbon cloth or paper onto which a mixture of carbon black and PTFE is cast and compressed.
In an electrode that supplies air as a reactant, oxygen must diffuse through the backing in order to reach the catalyst layer. This diffusional barrier lowers the effective concentration of oxygen at the catalyst layer with concomitant reduction in fuel cell performance. U.S. Pat. No. 4,129,685 to Damiano teaches the use of porous flow-fields in place of ribbed flow-fields in phosphoric acid fuel cells. Relatively thick layers of fibrous carbon paper or bonded particles are incorporated into the design of the gas-diffusion style electrodes conventionally used in phosphoric acid fuel cells. In this case, the carbon paper component is actually the flow-field because the reactants are forced to flow laterally through this component.
One of the difficulties with existing porous flow-field technology is the inherently high pressure drop that is generated when appreciable amounts of gas are forced through large electrode area porous flow-fields. A conventional solution is to use thick porous flow-fields. But the use of thicker flow-fields results in thicker unit cells, i.e., lower power densities. Further, the gas flows may tend to channel, especially in the case of PEM fuel cells, where water may accumulate and block off active regions. Finally, the high current density region is along the edge where the fresh reactant stream enters the cell, resulting in uneven heating of the cell and corresponding problems in cooling the cell.
These problems are addressed by the present invention. Accordingly, it is an object of the present invention to provide a porous flow field with reduced pressure drop through the flow field.
It is another object of the present invention to minimize water accumulation within the flow field.
One other object of the present invention is to maximize access of reactants to the catalyst layers.
Yet another object of the present invention is to provide for the use of very thin backings to minimize the gas diffusional barrier.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.